Internet usage is increasing dramatically across the world. Wireless connections starting with a wireless local area network (WLAN) are attractive in many environments where labor/construction costs are high. Historically, WLANs were viewed as a niche market with proprietary protocols, high costs, and unrealized performance. With the adoption of IEEE 802.11 standards, WLANs now offer a viable alternative to wired LANs, as evident with the explosive growth over the past few years. Both large and small companies have or plan to offer solutions based on IEEE 802.11b (WiFi™), 802.11a and 802.11 g. For wide spread adoption, issues in the form of security, higher speeds, and increased radius of operation will need to be addressed.
WLAN Overview
The adoption of 802.11 standards made possible increased speeds, interoperability between systems, and cost reductions that made WLAN a feasible alternative. Companies like Lucent, Intersil, Cisco, 3COM, Texas Instruments, Microsoft, and Intel have or have announced products supporting the IEEE 802.11 standards. The 802.11 standards define the physical layer (PHY) and media access control layer (MAC); since these layers are based on 802 Ethernet protocol and CSMA/CA shared media techniques, any LAN application, network operating system, or protocol (such as TCP/IP) will run on a 802.11 compliant WLAN.
The WLAN market is comprised of several technologies all competing with different techniques and performance characteristics.
                HomeRF and Home Rf 2.0 (WBFH)        IEEE 802.11b (DSSS)        IEEE 802.11a (OFDM)        IEEE 802.11g        HiperLAN/2        MMAC        
At the moment, the focus of the standard is on ether the 2.4 GHz band known as 802.11b or the 5 GHz band known as 802.11a. The supported data rates are up to 11 Mbps for 802.11b and are up to 54 Mbps for 802.11a. Products which conform to the 802.11b spec will in most cases work together and interoperate with ease. Essentially, the 802.11b or 11a standard provides open, asynchronous networking that requires a distributed control function.
Much like base stations for cellular technology, WLANs use an Access Point (AP) to provide wireless access to mobile terminals (MTs) or other devices in the network. AP is a cheap version of the base station for cellular technology and plays a very important role in WLAN. These APs are either connected to other APs, to other wired networks such as Ethernet, or connected to a broadband access medium such as DSL, cable, T1, etc.
IEEE 802.11b
The IEEE 802.11b operates in the unlicensed 2.4 GHz band. This standard permits two (2) distinctive types of transmission for data, Frequency Hopping Spread Spectrum (FHSS) and Direct Sequencing Spread Spectrum (DSSS). With the number of products and companies supporting DSSS, it has become the predominant standard for IEEE 802.11b. A raw data rate of 11 Mbps, 5.5 Mbps, 2 Mbps, or 1 Mbps is specified with a range of 100 meters.
Conventional configurations include single carrier, single receiver (Rx) and single transmitter (Tx) deploying a single omni-directional or dual dipole antenna. This is a simple and low cost solution. 802.11b is the predominant solution available on the market today. Since lower cost RF components may be used to achieve the requirements of 802.11b, the system cost has contributed to rapid growth.
Fundamental wireless channel impairments such as multipath (delay spread, temporal and frequency fading), interference, and noise greatly reduce the radius of the system. In most indoor environments, the 11 Mbps data rate is not achievable at 50 meters.
IEEE 802.11a
For higher speeds, companies are looking at IEEE 802.11a with 54 Mbps data rate. 802.11a uses a technique called Orthogonal Frequency Division Multiplexing (OFDM). OFDM sends multiple data streams simultaneously over separate radio signals in the less congested 5 GHz radio band, which has three (3) times the available spectrum. However, as the number of devices utilizing this band increases, congestion will also become an issue.
Although 802.11a offers a high data rate of 54 Mbps, a fundamental difference between 2.4 GHz and 5 GHz is the transmission range and corresponding coverage area. All things being equal, a higher frequency band will transmit a signal a shorter distance than a lower frequency band. The actual range at 54 Mbps in many instances may be less than 20 meters. This is of particular significance when considering the number of access points (APs) required for a similar area of coverage using 802.11a compared to 802.11b.
Barriers—Interference/Noise
Given the high cost of licensed spectrum, typical WLAN systems utilize either the 2.4 GHz or 5 GHz unlicensed (free) spectrum. As such, other devices and technology like microwave ovens, Bluetooth, satellite systems, and proprietary applications utilizing these unlicensed bands has created an overcrowding situation that will only get worse. A fundamental concern for all WLAN is the interference and noise between devices operating within the same spectrum.
Interference and noise may be viewed in two (2) types; in-band interference and out-of-band interference. Out-of-band interference or noise may be filtered out using the analog section of the receiver. In-band interference would include such time-varying impairments as multiple access interference and multipath conditions. Because the transmitted signal may take multiple paths in reaching the receiver, signal processing is required to address the delay spread, temporal and frequency fading.
Generally speaking, as the noise and interference increases the decipherable signal radius decreases. As a result, additional APs are required to complete coverage for a given area increasing costs and contributing to more interference.
Security
In a survey sponsored by Microsoft, security was the primary issue concerning companies implementing WLANs. The 802.11 standards address the issue in a couple of ways: Extended Service Set ID (ESSID) and Wired Equivalent Privacy (WEP). For ESSID, all mobile units associate themselves with an AP. This type of protection is limited since some products allow the mobile unit to attach to any AP, while others allow the user to browse and dynamically attach to a network.
WEP is a shared-key encryption mechanism option under 802.1 that employs either a 40-bit or 128-bit encryption using the RC4 algorithm. Unfortunately, many vendors have only just begun to implement this feature and it still relies on manual key distribution.
Multi-antenna Technologies
Multi-beam wireless antenna systems are described in a number of references, including the following:    U.S. Publication No. 2001/0036843 to Thompson;    U.S. Pat. No. 6,351,499 to Paulraj et al.;
Furthermore, the use of multiple antennas for WLANs are also mentioned in an article entitled “Technologies and Performance for Non-Line-of Sight Broadband Wireless Access Networks” by Gesbert et al. in IEEE Communications Magazine, April 2002.
The above materials are hereby incorporated by reference.
Using multiple antennas at one or both ends of a wireless link can significantly increase a bit rate. By using multiple antennas at both the transmitter and the receiver, a matrix channel is created in which transmitting occurs over several independent spatial “dimensions” or “modes” within the same time frequency slot at no additional power expenditure.
In the art, this technique is referred to as spatial multiplexing (SM). As Gesbert et al. explain, a data rate can be scaled in accordance with a number of antennas employed.
In operation, a high-data rate signal to be transmitted is first multiplexed into multiple bitstreams. These bitstreams are then transmitted simultaneously using multiple antennas. This causes the independent signals to be mixed in the channel since they occupy the same time and frequency resource.
At the receiver, the multiple received signals are separated, and individual data streams are demultiplexed to yield the original high rate signal. The separation is made possible by the fact that each transmit/receive antenna effectively sees a very different channel because of extensive multipath effects.
While such references generally describe the utility of multi-beam signal processing, they do fail to describe embodiments which would be compatible with conventional 802.11x based protocols (i.e, 802.11a, 802.11b, 802.11g, etc.) to enhance a conventional operating range of access points systems, and/or which could be efficiently implemented in integrated circuit solutions.
There is also a strong need for wireless systems which can handle situations in which the separation of incoming signals is very small, such as occurs in typical office space environments.
Furthermore, wireless security remains a significant problem. Any additional measures which can be used to “encrypt” a data transmission are extremely valuable in wireless applications.